(a) Field of the Invention
The present invention relates to process for production of surface-active carbohydrate fatty-acid esters, in particular, a low temperature, solvent-free trans-acidolysis reaction between carbohydrate ester of low molecular-weight carboxylic acid and free fatty acid under reduced pressure and in the present of acid catalyst.
(b) Description of the Prior Art
Carbohydrate fatty-acid esters are non-irritant, nonionic surfactants with excellent biodegradability properties. They are used to solubilize membrane proteins and to formulate many grades of detergents, pharmaceutical, food and cosmetic products. Carbohydrate fatty-acid esters are also used as therapeutic agents. U.S. Pat. No. 5,739,117, Yokoyama and Yoneda, issued April 1998, describe the use of glucose esters as cerebral metabolism improving agent. Carbohydrate esters can also be used for treating gallstones (U.S. Pat. No. 4,264,583), colonic disorders (U.S. Pat. No. 5,840,860) and hypercholesterolemia (U.S. Pat. No. 4,241,054). They are also known to exhibit anti-microbial and insecticidal activities. But the synthetic methods for carbohydrates fatty-acid esters are faced with many limitations.
Processes for the synthesis of sucrose fatty acid esters in solvent and non-solvent environments; following batch-wise or continuous reaction regimes currently exist. Prior art is described in U.S. Pat. Nos. 4,614,718, 4,927,920; 4,966,966; 4,968,791; 4,996,309; 5,043,438; and 5,908,922. The conventional methods, described in the prior, art for producing carbohydrate fatty esters have however encountered the following disadvantages and limitations:                (1) The transesterification method, currently favoured by most industrial producers of carbohydrate fatty esters, is a reversible process with poor conversion equilibria because the substrate sugars and fatty acid moieties are immiscible due to polarity differences. Thus, low product yield characterize the processes;        (2) Finding mutual solvent with good safety credentials. Mutual solvents, such as pyridine, N, N-dimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide (DMSO), chloroform, benzene and toluene, are required to solubilize the substrates. These mutual solvents are toxic and cannot be removed to the level compatible with current regulations, thus limiting product applications.        (3) Complex and costly product purification procedures.        (4) Thermal degradation and isomerization of the reactant carbohydrate at temperatures above 100 degree C., which is normally required in most known processes.        (5) High catalyst load, especially in enzymatic processes.        
U.S. Pat. No. 5,945,519 describes a process for the solvent-free preparation of sucrose fatty acid esters and their mixtures with nonsugar polyol fatty acid esters. In this process, a sucrose is reacted with one or more fatty acid alkyl esters of a chain length of 6 to 20 carbon atoms at temperatures between 120 to 160 degree C. and the reaction mixture is then reacted at reduced pressure with fatty acid alkyl ester, and then filtered without addition of solvent.
To address the problem associated with the use of toxic mutual solvents, U.S. Pat. No. 4,996,309 discloses a process for preparing sucrose fatty acid esters by reacting sucrose and a fatty acid alkyl ester in an aqueous reaction system in the presence of an alkaline catalyst. The catalyst introduces large amounts of soap in the product. Separation and purification is thus made cumbersome and expensive, involving a reverse osmosis and an ultrafiltration step. Even then, only 70% product purity is normally attainable.
U.S. Pat. No. 5,872,245 describe a continuous process for the synthesis of sucrose fatty acid esters by reacting sucrose in limited amount of methanol, as “carrier” solvent, with fatty acid methyl ester in the presence of stationary transesterification catalyst and mechanical emulsification. The process employs heavy metal catalysts, such as Zn, Cu, Sn and Pb, in addition to catalytic amount of sodium hydroxide. Product separation is achieved by density differentiations.
The process does not however address the color and antifacts introduced by the presence of sodium hydroxide and heat. Moreover, the effective density differentiation of the product mixture of more-than-six similar products and reactants results in a long holdup time in the process cycle. The formation of mono-, di-, tri- and polyester cannot be selectively controlled, there is therefore a stockpile of “over esterified” polyols formed in the process.
It is a principal object of the present invention to provide a solvent-free trans-acidolysis process for preparing surface-active carbohydrate fatty-acid esters, which obviates the above drawbacks and/or limitations by adding no solvent during reaction, driving esterification rapidly to polyester formation and, when necessary, partially deacylate to form target products.